The City of San Diego's water department received preliminary approval Wednesday to expand and move its experimental aquaculture plant from Mission Valley to the San Pasqual Valley, where treated sewage water will be sold for agricultural use.
The City Council's Public Services and Safety Committee voted unanimously to approve the relocation of the water hyacinth plant, making possible the first large-scale commercial reuse of recycled sewage water.
The city's aquaculture plant--situated just north of Interstate 8, between Camino del Rio North and the San Diego River--now treats 300,000 gallons of sewage a day, said Frank Maitski, a senior civil engineer in the city's water department.
The California Department of Transportation buys 40,000 to 50,000 gallons of the treated sewage a day to water highway landscaping at peak times during the summer, he said.
The rest of the sewage treated at the experimental Mission Valley plant is simply dumped back into sewers, he said.
But the $20.5 million federal grant, which pays for most of the construction and operation of the aquaculture plant, anticipates that the city will eventually expand the plant to treat 1 million gallons daily, Maitski said.
That could be done in Mission Valley, but it would require the realignment of a road and leave the city hard-pressed to find more customers to buy the recycled water for acceptable uses, he said.
So water department officials decided last year it would be better to move the plant to city property along Highland Valley Road in the San Pasqual Valley, about 2 miles from Rancho Bernardo. The idea was that local farmers could use the treated water for irrigation. Maitski said a local nursery has already agreed to buy the million gallons of treated sewage at about 90% of the cost of normally potable water.
"Besides being an experimental plant, we want to turn it into a reclamation plant as well," Maitski said. "This is our opportunity to save some capital costs by relocating it to an area where it could be used as a reclamation plant as well."
The city will also begin making use of the water hyacinths themselves, said Maitski. Now, old hyacinths are plucked from the Mission Valley ponds and sent to the landfill.
But, when the San Pasqual Valley facility is built, the city plans to squeeze the hyacinths for their juices and then convert the plants into a soil additive. The hyacinth juice will be put into a special digester to produce methane gas, Maitski said.
If approved by the full City Council, construction of the new aquaculture plant will begin early next year and will cost $12.4 million, $5.8 million of which will be paid by the federal government. The remainder--$6.6 million--will be picked up by the city, he said.
Part of that cost will be offset by the fact that the new San Pasqual Valley plant will be treating 1 million gallons of sewage produced by the Rancho Bernardo community, Maitski said. Otherwise, he added, that sewage would have to be pumped to Escondido and treated at a conventional waste-water plant there.
For years, aquaculture has been praised by city officials as the simple solution to the region's most pressing water problem: How to keep the cost of treating sewage down while weaning San Diego from its supply of imported water.
The idea was to fill shallow ponds with sewage that has gone through the primary treatment stage, which removes 40% to 60% of the suspended solids. Then, marine animals and microorganisms are allowed to nibble away at the remaining suspended solids, yielding water that is 90% free of sewage. The microorganisms live on the roots of the hyacinths, which cover the surface of the ponds.
Hopes for the natural technology have been deflated, however, because the city's experiment has shown that the microorganisms weren't gobbling the sewage fast enough. The city was forced to retrofit its Mission Valley ponds with air injectors to speed the treatment process--a costly step.
Besides, the experiment showed that aquaculture is relatively land-intensive, leaving little hope that it can be used for secondary sewage treatment of the 180 million gallons of sewage that passes through the Point Loma plant. Aquaculture also attracts mosquitoes and gives off odors.
But water department officials stress that the hyacinth plant is not a total disappointment, and that it can be used on a limited scale for such projects as irrigation in the San Pasqual Valley.
Although water produced by the hyacinth plant can be used for agricultural purposes, it is not potable and, therefore, is unfit for human consumption. It must be run through a reverse osmosis process before anyone can drink it.